That was unfortunate timing... He should have filed after moving using new address.
USPS forwarding works 50% of the time.
I was moving as my I-751 got approved. Nothing worked. USPS forwarding, mail intercept, hold mail... All failed and GC showed at old address.
If he has connection to somebody living at / managing old address, he should reach out and see what mail they received.
It's possible to USCIS to send further notices to new address after they process AR-11. It's also possible for them to keep sending everything to the old address.
I guess son put old address in I-751?